Periodicity analyser for a quasi-periodic signal including a detection circuit

ABSTRACT

A periodicity analyzer as a pitch meter for a quasi-periodic signal, for example speech, equipped with a detection circuit which converts the signal into a plurality of pulses the positions of which correspond to those of the peaks of the quasiperiodic signal, which detection circuit includes a peak detector the time constant of which is substantially equal to the shortest period of the quasi-periodic signal to be detected, while the pulses are applied to a cascade arrangement of a plurality of period filters the greater part of which have mutually different characteristic time durations. The greater part of the time durations form an ascending series. With each period filter there is associated an auxiliary circuit comprising a stretcher and an input AND gate in order to distinguish the main periods from the sub-periods. Thus, by measuring the spacings between the peaks in the speech signal to be examined, which have been transformed into pulses, the pitch is directly determined. This is effected by means of a digital analog converter. This periodicity analyzer can advantageously be used in a vocoder as a pitch detector.

United States Patent Wlll ms 51 July 11, 1972 [54] PERIODICITY ANALYSER FOR A Primary Examiner-Kathleen H. Claffy QUASI-PERIODIC SIGNAL INCLUDING Assistant Examiner-Jon Bradford Leaheey A DETECTION CIRCUIT Attorney-Frank R. Trifari [72] Inventor: Leonardus mnelsltus Willems, Emmasin- [57] ABSTRACT gel, Eindhoven, Netherlands A periodicity analyzer as a pitch meter for a quasi-periodic AsslBneei l" w New York signal, for example speech, equipped with a detection circuit [22] fl July 6, 1970 which converts the signal into a plurality of pulses the positions of which correspond to those of the peaks of the quasil l PP N04 52,639 periodic signal, which detection circuit includes a peak detector the time constant of which is substantially equal to the 30 Foreign n u PM). shortest period of the quasi-periodic signal to be detected,

while the pulses are applied to a cascade arrangement of a plu- July 9, 1969 Netherlands ..69 10496 m of period fihers the greater pan f hi h have mutuany different characteristic time durations. [5 l] The greater part of the time durations form an ascending se- [58] Field of Search l 79/ l SA; 324/ l 88, 77 A, 77 B; fits ith each period filter there is associated an auxiliary cir- 307/232, 233, 234 cuit comprising a stretcher and an input AND gate in order to distinguish the main periods from the sub-periods. Thus, by [56] References Cited measuring the spacings between the peaks in the speech signal to be examined, which have been transformed into pulses, the UNITED STATES PATENTS pitch is directly determined. This is efiected by means of a 3,020,344 2/l962 Prestigiacommo ..179/| SA digital analog converter- 3,469,034 9/ I969 Steward ..l79/| SA This periodicity analyzer can advantageously be used in 3 3,513,260 5/l970 Hellwarth .....l79/l SA vocoder as a pitch daemon 3,499,986 3/l970 Focht ..l79/l SA OTHER PUBLICATIONS 1 1 L. 0. Dolansky, An Instantaneous Pitch-Period Indicator, J.A.S.A. 1/1955 Vol. 27, pp. 67- 72.

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RECTIFIER LRF i I I I I I PERIODICITY ANALYZER PERIODICITY ANALYSER FOR A QUASI-I'ERIOIHC SIGNAL INCLUDING A DETECTION CIRCUIT Determining the pitch of sound is very important for analysis-synthesis telephony systems and for phonetic research.

Because since several years the opinion has been gaining ground that the human ear perceives the pitch of a sound signal by measuring time differences between peaks which occur in the fine structure of this signal (see .I.Acoust. Soc. Am., 34. 9. pages 1.224 1,229), several pitchmeters in the form of periodicity analyzers have been developed according to this principle.

The invention relates to a periodicity analyzer for a quasiperiodic signal, for example speech. including a detection circuit which converts the signal into a plurality of pulses the positions of which correspond to those of the peaks of the quasi-periodic signal.

Such a periodicity analyzer is known from J. Acoust. Soc. Am., 27, I955. pages 67 72. This periodicity analyzer includes a peak detection circuit having a time constant the value of which exceeds the longest period of the sound signal to be detected. By this large time constant the measuring range is reduced. it has also been found that with rapid decreases in amplitude of the sound signal this periodicity analyzer cannot follow the decrease so that a few periods are skipped.

The periodicity analyzer according to the invention is characterized in that the detection circuit includes a peak detector the time constant of which is substantially equal to the shortest period of the quasi-periodic signal to be detected. the pulses being applied to a cascade arrangement of a plurality of period filters most of which have mutually different characteristic time durations.

Thus the above-mentioned disadvantages are avoided. Because of the smaller time constant of the peak detector all the amplitude variations and in particular the amplitude decreases can be completely followed so that all the main periods will be clearly present in the pulsatory signal.

However, even in this case the pulsatory signal may contain sub-periods. By passing this signal through a so-called period sieve which comprises the said cascade arrangement not only the main periods but also any subperiods are indicated.

Each period filter comprises an electronic circuit which on reception of a starting pulse delivers a rectangular output signal the duration of which is equal to the sum of the characteristic time of the period filter and the time which elapses between the starting pulse and any succeeding pulse. provided that this succeeding pulse occurs within the characteristic time duration, i.e. on reception of a train of pulses the spacings between which are smaller than the characteristic time of the respective period filter a continuous signal is delivered.

The quasi-periodic signal converted into pulses is applied to the cascade-connected period filters. The first period filter delivers signals the duration of which corresponds to the characteristic time duration associated with this period filter. The starting point of each rectangular signal actuates the next period filter with the result that likewise rectangular signals are produced having a time duration equal to the characteristic time of this second period filter. This procedure is repeated until the spacing between two successive initial pulseswhich still occur in the rectangular signal as starting points at the instants is smaller than. or equal to, the characteristic time duration 1,. of the k" period filter. This filter then will deliver a continuous signal, which is an indication of the presence of a period having a time duration substantially equal I...

Since the next period filter (k 1) receives a continuous signal without starting points, this filter and the succeeding filters will not deliver signals.

Operation of the periodicity analyzer according to the invention will be particularly effective if the period filters are connected in the order of increasing values of the characteristic times. Satisfactory results are obtainable when these time duration do not differ from one another by more than 1/10.

In order to distinguish all main periods from the sub-periods in the quasi-periodic signal to be analyzed, each period filter is given an auxiliary circuit. This comprises a stretcher which preferably is designed as a period filter to which an input AND gate is added which is connected to the outputs of the associated and the preceding period filters.

Thus, the stretcher delivers one or more signals. the time duration of the or each such signal being equal to the characteristic time duration of the associated period filter, namely the period filter the characteristic time duration of which is just slightly shorter than the period to be measured.

An embodiment of the invention is characterized in that there is included in the circuit connecting the input AND gate and the output of the associated period-filter an AND gate the input of which is also connected to an OR gate which is connected to the preceding stretcher, the succeeding stretcher and the stretcher itself. This ensures that when a given stretcher is operative both the preceding stretcher and the succeeding stretcher will become operative as soon as they receive a pulsatory signal from the associated period filters.

At the starting of the periodicity analyzer before any stretcher is operative it is desirable to apply a starting signal to the said OR gate so that all the stretchers can be started.

Further. in order to ensure a continuous indication by the periodicity analyzer it is of particular advantage for the characteristic time duration of a stretcher to be fractionally greater than that of the associated period filter.

It has been found that with a continuously falling pitch which results in a continuously increasing period the signal from the stretchers do not overlap, which is indesirable for the indication. if a period filter is arranged to be started by the two preceding period filters, this defect is eliminated.

Hence, in another embodiment of the invention there is included between two successive period filters an OR gate the input of which is connected to the outputs of two period filters preceding the OR gate.

Each period filter and each stretcher comprise an electronic circuit which includes at least a first and a second transistor, the collector of the first transistor being connected to the base of the second transistor, while the emitters of the two transistors are interconnected and a capacitor is included between the base of the first transistor and the collector of the second transistor, the base of the first transistor being connected through a differentiating network to an input terminal and the collector of the second transistor being connected to a Schmitt trigger.

A pulse or rectangular signal which is received is differentiated and then applied to the base of the first transistor. As a result, the first transistor is cut off and the second transistor becomes conductive. The capacitor connected between the two transistors which capacitor has been charged in the inoperative condition rapidly discharges. The changeover level of the Schmitt trigger is passed so that it delivers a signal. Subsequently to the input pulse the first transistor becomes conductive and the second transistor is cut off, and the capacitor charges. The input voltage of the Schmitt trigger rises accordingly, the change-over level is passed and the Schmitt trigger stops. Thus. the Schmitt trigger has delivered a rectangular signal the duration of which characteristic time duration) depends upon the charge time of the capacitor. which time in turn is determined by the value of the capacitor and the values of the resistors in the collector circuit of the second transistor.

The periodicity analyzer includes an indicator comprising a plurality of indicator elements which are each connected to a stretcher. An indicator element substantially comprises an AND-gate the input of which is connected to the output of the associated stretched and also, through an inverting amplifier, to the output of the succeeding stretcher.

This prevents that, when the period to be measured passes from one interval to the adjacent interval. for a shon time two stretchers will simultaneously be operative with a consequent double indication.

In an advantageous embodiment of the invention the indicator elements are connected to a digital-analog converter.

A periodicity analyzer according to the invention can be used to advantage in a vocoder.

For a vocoder equipped with a pitch-meter it is of importance to operate in a large frequency range with an exact reproduction of the periods present in the speech. A vocoder equipped with a periodicity analyzer according to the invention satisfies the said requirements.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows a signal transformation circuit,

FIG. 2 shows the various signals at several points of this circuit,

FIG. 3 shows a periodicity analyzer according to the invention in its simplest form,

FIG. 4 shows the signals in various component parts of this circuit,

FIG. 5 shows the periodicity analyzer of FIG. 3 extended to include stretchers,

FIG. 6 shows a practical embodiment of an analyzer according to the invention,

FIG. 7 shows various signals obtained in the case of a regular input signal comprising two periods,

FIG. 8 shows similar signals in the case of an irregular input signal,

FIG. 9 shows similar signals in the case of several periods having ascending values,

FIG. 10 is a circuit arrangement of a period filter used in the periodicity analyzer, and

FIG. 11 shows schematically a vocoder circuit using the periodicity analyzer according to the invention.

FIG. I shows a circuit arrangement in which the applied quasi-periodic signal in this case sound is converted into a plurality of pulses the positions of which correspond to those of the peaks which occur in the quasi-periodic signal.

The speech signal is applied to the input terminals 1 and 2. In ut terminal I may be connected to a microphone the signal of which is amplified by a microphone amplifier 3. The input terminal 2 may be directly connected to a magnetic taperecorder commonly used in phonetic research.

The original speech signal shown by a curve in FIG. 2 is passed through a selection switch 4, a grounded regulating potentiometer 5, 6 and an amplifier 7 to a rectifier circuit 8. The signal at the output of the rectifier then will have the form shown by a curve 21 in FIG. 2.

Depending on the nature of the initial signal the rectified signal may be passed by means of a switch 11 through a lowpass filter l0 having a cut-off frequency of I000 Hz and an attenuation of about 36 db per octave.

The signal then will have a form corresponding to a curve 22in FIG. 2.

The peaks in the signal will then be additionally enhanced in a peak enhancer l2 comprising a peak rectifier and an adder. The peaks obtained by peak rectification are added to the initial signal. The resulting signal has the form of curve 23 in FIG. 2.

The value of the time constant of the peak rectifier plays a very important part. If this time constant is large, only a single pulse will be formed per cycle, which would be sufficient, provided that amplitude variations and especially amplitude decreases could be followed. However, this is only possible of the time constant is small, i.e. about equal in value to that of the shortest period in the signal.

The last stage of the signal transformation consists in the formation of pulses the positions of which corresponds to the positions of the enhanced peaks. This pulsatory signal (curve 24in FIG. 2) is produced in a pulse shaper 13.

The pulsatory signal, which contains the periodicity of the initial quasi-periodic signal, is applied to a period sieve. This period sieve is shown in its simplest form in FIG. 3. It comprises a cascade arrangement of period filter in the form of digital elements. In each element the same operation is performed. The operation is as follows. On reception of a pulse a period filter delivers a rectangular signal having a given characteristic duration r,,. If the signal consists of several pulses the intervals between which exceed the characteristic time I the output signal will consist of a series of rectangular signals of equal duration n, the starting points of which correspond to the positions of the pulses of the input signal. If the signal includes several pulses the spacings between which are smaller than the characteristic time r,,., the duration of the rectangular signal will be increased by this time. If the spacings between all the pulses are smaller than the time r a continuous output signal will be obtained.

When a period filter receives several rectangular signals instead of pulses, its operation will be the same, however, it is actuated by the starting points of the rectangular signals.

The cascade arrangement of FIG. 3 comprises a series combination of period filters 40, 42, 44, S0. The characteristic values of these filters form a ascending series. In practice good results have been obtained with mutual differences of 6 percent. To illustrate the operation of this period sieve in its simplest form, FIG. 4 shows the various output signals at output terminals 41, 43, 45, 53.

In this Figure it has been assumed for greater clearness that the characteristic time of each period filter is 20 percent greater than that of the preceding filter.

To the input 24 of the period sieve of FIG. 3 there is applied a regular pulsatory signal containing two periods which in the uppermost curve 24 of FIG. 4 have been designated by a and b This signal is processed by the first period filter 40. Processing the first pulse produces a rectangular signal having the characteristic duration I This rectangular signal has disappeared before the second pulse is received, since the time 1,, is shorter than the period a so that a second identical rectangular signal of duration r is produced. This process is repeated when after the time b an identical train of pulses is applied to the period filter 40. Thus, there is produced at the output terminal 41 an output signal which corresponds to the curve 41 of FIG. 4 (the reference numerals of the signals produced in the circuit elements correspond to those of the points in these circuits at which these signals appear).

The output signal is then processed in the second period filter 42.

The starting points of the rectangular signals, the positions of which correspond to those of the pulses in the initial signal 24, render this filter operative. There are produced rectangular signals equal in number to those of the curve 41 but having a duration which is 20 percent greater then n This time however, still is shorter than the period a.

The output signal 43 then is processed by the period filter 44. Since the characteristic time I of this filter is slightly greater than the period a, the starting point of the second rectangular signal restarts the period filter 44, and the signal already produced has its duration increased by a time equal to the characteristic time I (20 percent longer than 1, Thus there appears at the output terminal 45 a train of single rectangular signals with spacings equal to the period b.

The period filter 46 is started by the starting point of this "increased" rectangular signal and delivers a rectangular signal having a time duration I which in turn is 20 percent longer than t This process is repeated by the succeeding filters up to and inclusive of the filter 56 the characteristic time t of which is slightly shorter than the period b. The characteristic time 1 of the period filter 58 is longer than the period b, i.e. the starting point of each of the rectangular signals of the curve 57 will start the period filter 58 before the output signal has disappeared. There will be produced a continuous output signal as shown by the curve 59. The starting point of the output signal 61 is produced, which signal comprises a single rectangle having a characteristic time r In the succeeding period filters the same process is repeated with the difference that the time of each successive signal is increased by percent.

From the above it will be seen that the cascade arrangement of the period filters 40, 42, 60, 62 successively removes all the period in the input signal in the order of their lengths. In the case described, first the period a has been removed by the period filter 44 and then the period b by the period filter 56. Both filters approximately give the time durations of the periods a and b with a maximum error of 20 percent.

It will also be seen that all the periods which have been indicated by a pulse are removed and hence the sub-periods also which have been produced owing to the small time constant of the pulse shaper at the occurrence of amplitude decreases in the quasi-periodic signal.

In order to enable these sub-periods to be clearly distinguished from the main periods, in the circuit arrangement shown in FIG. 5 there has been added a stretcher to each period filter.

The stretchers are designed as digital circuit elements similar to the period filters, and they have characteristic times substantially equal to those of the associated period filters. However, there is a difference: the input of each stretcher includes an AND gate which is connected to the output of the preceding period filter and also to the output of the associated period filter.

The period filters 42, 44, 48, are associated with stretchers I42, 144. 148, respectively, the stretcher 142 being connected through a lead 241 to the output 41 of the period filter 40 and through a lead 242 to the output 43 of the period filter 42; similarly, stretchers 144 and 146 are connected through leads 243 and 245 to outputs 43 and 45 and through leads 244 and 246 to outputs 45 and 47, respectively.

Each stretcher is started in the same manner as is the period filter, i.e. by a pulse or by the leading edge of a rectangular signal. However, starting is effected only if there is applied to the input of the respective AND gate a second signal from the output of the associated period filter. This is the case when the associated filter delivers a signal of a duration which exceeds its characteristic time and is about equal to a multiple of the duration of the period being sieved.

In the embodiment shown in FIG. 4 this happens to the stretcher 144.

The period filter 42 delivers a signal comprising two rectangles (curve 43); the period filter 44 delivers a signal comprising a single rectangle (curve 45). At the starting point of the second rectangle of the curve 43 an output signal of the period filter 44 is available. At this instant the stretcher I44 is started and at its terminal 145 delivers a rectangular signal of duration I, substantially equal to the duration of the period 0.

Similarly the period b is measured by the stretcher I58 becoming operative.

By connecting to each output terminal 143, 145, 147, a simple indicating device the simplest way is to connect an incandescent lamp to each terminal a direct visible indication is obtained of the various periods present in the signal to be analyzed.

FIG. 6 shows part of the period sieve of a very effective periodicity analyzer. It comprises a cascade arrangement of 36 sections of identical design. Each section substantially comprises a period filter and an associated stretcher, as is shown in FIG. 5. However, the circuit arrangement is extended to include various gate circuits.

FIG. 6 shows 3 sections 1, II and III of the 36 sections of a complete sieve.

Viewed from the inputs of the period sieve the charac teristic values of the period filters 42, 44, 46, etc., and of the stretchers 142, 144, 146, etc., increase by an equal percentage. Very good results have been obtained with an increase of 6 percent.

The input of each stretcher is provided with a gate circuit. Leads 241, 243 and 245 are similarly connected to the outputs of the preceding period filters 41, 43 and 45 by means of connecting leads 541. 543 and 545, respectively.

The other parts 242, 244 and 246 of the input circuit of the stretchers 142, 144 and 146 are provided with AND gates which are directly connected through leads 443, 445 and 447 to the outputs 43, 45 and 47 of the associated period filters 42, 44 and 46, respectively.

The AND gates 242, 244 and 246 are also connected to OR gates 442, 444 and 446, respectively.

The OR gate 442 receives the following signals:

a general starting signal g,

the output signal of the preceding stretcher 540, the output signal of the associated stretcher 542, the output signal of the succeeding stretcher 544.

Similarly, the OR gates 444 and 446 receive the output signals of stretchers 542, 544 and 546 and from 544, 546 and 548, respectively, and furthermore the signal 3.

A further modification of the period sieve of FIG. 5 is provided in the input circuit of each period filter.

Between each pair of successive period filters there is connected an OR gate. The outputs 43, 45 and 47 of these period filters (42, 44 and 46) are connected to the inputs of the OR gates 642, 644 and 646, respectively. These inputs are also connected through leads 541, 543 and 545 to the outputs 41, 43 and 45 of the preceding period filters 40, 42 and 44, respectively.

The output circuit of each stretcher, the input signal to which is to be used as an input signal for an indicating device, has also been modified by the interposition of an AND gate. The AND gates 343, 345 and 347 are directly connected to the outputs of the stretchers 142, I44 and 146 through leads 642, 644 and 646, respectively, and also through leads 643, 645 and 647 to the outputs of the succeeding stretchers 544, 546 and 548, respectively. An inverting amplifier A is included in each of the leads 643, 645 and 647.

The operation of the embodiment of the period sieve shown in FIG. 6 will now be described with reference to some practical examples. In the first example (FIG. 7) the signal to be analyzed is a quadri-periodic signal consisting of a simple regular pattern of two periods 0 and d. After being converted into pulses this signal is transformed by the first period filter of the period sieve to a rectangular signal each rectangle of which has a duration equal to the characteristic time of this filter. The next subsequent period filter is started by the starting points of the said "rectangles and in turn delivers a rectangular signal having a characteristic time which is longer by ID percent. In this manner, when the period filter 42 is reached a rectangular signal is produced which has a characteristic time r which still is smaller than the period c. This signal is designated by the curve 42 in FIG. 7. The period filter 44 then delivers a signal which is larger by 10 percent and the characteristic time of which is smaller than the period r. by less than l0 percent. The period filter 46 is started by the first starting point of the output signal of the period filter 44; however, the input signal the output signal of the period filter 44) contains a succeeding starting point within the characteristic duration 1 so that from this instant the period dfilter 46 remains operative for an additional time equal to the characteristic time 1, At this instant a continuous output signal from the period filter 46 will appear at the input of the stretcher 146, i.e. at the AND gate 246, so that this stretcher is rendered operative and delivers a rectangular signal having a characteristic time r t +A, where A is about 5 percent.

The preceding stretcher 144 and a succeeding stretcher I48 remain inoperative, since at the instants of the starting points the associated period filters 44 and 48 respectively do not deliver output signals.

Similarly the second period at is sieved out. After the signal has passed through a number of succeeding period filters the characteristic time of each filter exceeding that of the preceding filter by ID percent and reaches the period filter 68, the stretcher 168 will become operative, since at the instant of the second starting point the period filter 68 delivers a continuous output signal.

This example clearly shows that on passing from period c to period d two stretchers, 146 and 168, will simultaneously be operative. The output signal of the stretcher three rectangles the last two of which occur at as the output signal of the stretcher I68.

This is undesirable for the indicating arrangement. Hence the input circuit of each stretcher includes an OR gate (442, 444 and 446); this gate can receive a starting signal 3, the output signal of the preceding stretcher, of the succeeding stretcher and of the associated stretcher.

From FIG. 7 it can be deduced that when the stretcher I46 is operative the stretchers 144 and I48 are inoperative. Neither will there be a starting signal 3. This starting signal is delivered only when no other stretcher is operative (see the lowermost curve of FIG. 7).

At the starting point of the first rectangle there still will be a starting signal 3 at the OR gate 446, for the starting signal is switched off only at the instant at which the stretcher I46 becomes operative. The preceding and succeeding stretchers 144 and I48) are inoperative, and the associated stretcher (I46) has not yet been rendered operative (will now become operative). The starting signal renders the OR gate 446 operative so that a signal appears at the AND gate 246 and renders the stretcher I46 operative.

If the second rectangle in the output signal of the stretcher 146 should occur, there is no starting signal 3. because the stretcher 168 is operative. Thus, no signal appears at the OR gate 446, i.e. the stretcher I46 cannot start operating.

A signal which in FIG. 7 is designated by 146' and comprises a single rectangle will be produced at the output terminal I47. However, the output signal of the stretcher I68 appears unaltered at the output terminal 169.

The example illustrated in FIG. 8 relates signal having irregular periods.

These periods are in the following ratio: It l m 33 36.5 40.5.

As has been set forth with reference to FIG. 7, the periods are shifted out one by one in the order of ascending duration.

The period filters 62 and 64 have characteristic times shorter than the shortest period K. The characteristic time I is slightly longer than the period It and hence the stretcher 166 will become operative (in this example. the instant ofstarting is the fourth starting point from the beginning of the second starting point after the new period It).

The period filter 66 has a characteristic time slightly shorter than the period I. The next subsequent period filter 68 has a longer characteristic time, so that the stretcher I68 becomes operative. The period I is similarly indicated by the stretcher [70.

FIG. 8 shows that the stretcher 168 will first become operative. The output signal of this stretcher still exists when the stretcher I66 becomes operative. As will be seen, this is also the case for the stretcher I70, which will become operative a short time before the stretcher 168 becomes inoperative.

The output signals of the stretchers overlap. In order to prevent indicating arrows the output of each stretcher is provided with an AND gate.

In FIG. 6 the stretcher I42 is connected to an AND gate 343. This AND gate also receives, through the inverting amplifier A, the inversion signal from the next subsequent stretcher I44.

In general, the output signal ofth AND gate is I46 comprises the same time to a quasi-periodic N lli'l where V, the output signal of the n" stretcher and W, inversion of the output signal (n+l stretcher.

In the example of FIG. 8, the AND gate 167 will only transmit the input signal of the stretcher I66 when the first rectangle of the output signal of the stretcher I68 has terminated. The AND gate I67 stops prematurely at the instant at which the stretcher 168 starts a second time (second rectangle). The AND gate 169 stops at the instant at which the stretcher 170 becomes operative.

Not only the simultaneous operation of two stretchers, but also the occurrence of hiatuses between stretches is undesirable for a continuous indication.

The latter effect occurs with quasi-periodic signals the periods of which ascend with small differences. This illustrated in the example of FIG. 9. There are 4 periods: p, q, r and s with the following ratio: p q r F l 1.08: 1.16: L20. As will be seen, the relative difi'erences are comparatively small. However, this is of common occurrence in normal speech signals.

FIG. 9 shows that first the period p, then the period q and subsequently the periods r and s are sieved out. These periods are successively indicated by the stretchers I56, 158 and 160. A consideration of the output signals of these stretchers shows that the stretcher 156 is no longer operative when the stretcher I60 starts. The latter stretcher also stops before the next stretcher 142 starts. This gives rise to hiatuses between the output signals of the stretchers.

To prevent this effect each period filter is provided with an input gate (642, 644, 646 in FIG. 6) which is connected to both preceding period filters. Thus, a period filter will only operate when a signal is received from both preceding period filters (a pulse or starting point of a rectangular signal).

In FIG. 9 the signals produced by this circuit are designated by primes. It will be seen that only starting from the period filter 58 the output signals are different. The output signal 58' still persists at the instant at which the period q occurs, so that the stretcher I58 will become operative on termination of the first period q (see curve I58). This is just fractionally before the end of the output signal of the stretcher 156.

This also applies to the next stretcher I60, which will start at an earlier instant owing to the fact that at the starting instant the output voltage from the period filter 60 is present (see curve 60').

FIG. I0 shows the circuit diagram of a period filter. It substantially comprises transistors I and 2 and a Schmitt trigger 3 shown in block-schematic form.

The two transistors l and 2 are fed from terminals 4 and 5 by supply voltages +V and V, respectively. A collector resistor 6 of the transistor I is connected to earth at 7. The base of the transistor I is also connected to earth through a resistor 8; this base is further connected to the negative supply terminal 5 through a resistor 9. It is also connected to an input terminal 12 through a capacitor 10 and an inverting amplifier II. The emitter of the transistor l is connected to the supply terminal 5 and to the emitter of the transistor 2. The collector of the transistor 2 is connected to the positive supply terminal 4 through the parallel combination of a resistor 13 and of the series arrangement of a diode I4 and a resistor 15. A capacitor I6 is inserted between the collector of the transistor 2 and the base of the transistor I. The diode I4 is connected to an output I8 through the Schmitt trigger 3 and an inverting amplifier I7.

The period filter operates as follows.

In the operative condition the transistor I is conductive. The voltage at its collector is about equal to the negative supply voltage --V,. As a result the transistor 2 is cutoff. The capacitor 16 will now be charged through the diode I4 and the resistors 13 and I5 to a positive voltage substantially equal to XV,,. This voltage will also be set up at the input of the Schmitt trigger 3. The level at which the Schmitt trigger changes condition is 0 volts. The Schmitt trigger does not deliver a signal. If now a positive pulse or a positive rectangular signal appears at the input 12, this will be inverted by the inverting amplifier II and then differentiated by the capacitor 10 so that a negative pulse signal is applied to the base of the transistor 1. As a result the transistor I is cut off and the transistor 2 becomes conductive. permitting the capacitor 16 to rapidly discharge through the transistor 2 and the resistor 9. The voltage at the collector of a transistor 2 decreases and after the capacitor I6 has discharged is about equal to V,,, and the input voltage of the Schmitt trigger will have dropped correspondingly so that the change-over level of 0 volts is passed. Hence the Schmitt trigger becomes operative and delivers a negative signal. On termination of the input pulse the duration depends upon the values of the resistors 8 and 9 and of the capacitor 10, the transistor 1 becomes conductive again and the transistor 2 is cut off again, whereupon the capacitor 16 is charged through the resistors 13 and and the diode 14.

The voltage at the collector of the transistor 2 and hence the input voltage of the Schmitt trigger 3 rise. When the zero voltage level is passed, the trigger changes its condition and its output signal is broken off. A positive rectangular voltage is set up at the output 18 for a time depending upon the charge time of the capacitor 16. This charge time can be preset by the choice of the values of the resistors 13 and 15 and of the capacitor 16.

When another pulse appears at the input 12, the capacitor 16 discharges again and the afore-described cycle is repeated. If, however, during the charge time a succeeding pulse is applied to the base of the transistor 1, the capacitor 16 will begin to discharge at this instant; the Schmitt trigger 3 will not yet have changed its condition and from this instant the rectangular output voltage is prolonged by the characteristic time 1,, the charge time of the capacitor 16).

FIG. 11 shows schematically a channel vocoder including a periodicity analyzer. This channel vocoder the principle and operation of which are known (see M. Schroeder Vocoders: Analysis and Synthesis of Speech", Proceeding of the IEEE, 54, 3, pages 720 734), comprises 3 parts: an analysis part A, a transmission part B and a synthesis part C.

A speech signal received by a microphone l is divided by 14 band-pass filters 2 into a plurality of adjoining frequency bands of about 200 Hz each. Each frequency band is rectified by rectifiers 3 and is then passed through a low-pass filter 4 the cut-off frequency which is Hz. Each low-pass filter produces a signal which represents the time-varying mean amplitude of the respective frequency band. Together the l4 channel signals from the spectral information of the speech signal with a band-width of about 300 Hz. Thus the transmission bandwidth of the speech signal has been reduced to about one tenth part of the original speech signal.

A periodicity analyzer 5 according to the invention is associated as a pitch detector with the analysis part A of the vocoder. This analyzer determines the pitch of the speech signal.

After transmission by the part B the signal is again divided into the same number of channels. The information of the pitch detector 5 controls a switch 6 by which either a noise generator 7 or a pulse generator 8 is connected to the modulators 9 in the various channels. After the signals have been processed by the modulators 9 they are passed through bandpass filters to an adder 11 the output signal of which is fed to a loudspeaker 12.

WHAT IS CLAIMED IS:

1. A periodicity analyzer for a quasi-periodic signal, for example speech, comprising a detection circuit means for converting the signal into a plurality of pulses the positions of which correspond to those of the peaks of the quasi-periodic signal, the detection circuit comprising a peak detector the time constant of which is substantially equal to the shortest period of the quasi-periodic signal to be detected; and a cascade arrangement of a plurality of period filter means connected to the detection circuit, the filter means having mutually different characteristic time durations, each filter means comprising means responsive to input pulses separated by an interval greater than the characteristic time duration for providing output pulses having a pulse width equal to the time duration and responsive to a series of input pulses separated by an interval less than the characteristic time duration for providing an output pulse having a pulse width equal to the characteristic time duration plus the total time elapsed between the first and last pulse of the series.

2. A periodicity analyzer as claimed in claim I, wherein the characteristic time durations of the greater part of the period filters have ascending values.

3. A periodicity analyzer as claimed in claim 1. wherein the characteristic time durations of the period filters differ from one another by not more than 1 l in ascendin order.

4. A periodicity analyzer as c aimed in claim further comprising with substantially each of the period filters an associated auxiliary circuit means for distinguishing the main periods from the sub-periods of the quasi-periodic signal to be analyzed.

5. A periodicity analyzer as claimed in claim 4, wherein each auxiliary circuit consists of a stretcher of a design similar to that of a period filter and of an input AND gate connected to the output of the associated period filter and to the output of the preceding period filter.

6. A periodicity analyzer as claimed in claim 5, wherein in the circuit connecting the input AND gate to the output of the associated period filter there is included an AND gate the input of which is connected to an OR gate which is connected to the preceding stretcher, to the succeeding stretcher and to the associated stretcher.

7. A periodicity analyzer as claimed in claim 6, wherein to the said OR gate there is applied a starting signal at the instants at which none of the stretchers delivers an output signal.

8. A periodicity analyzer as claimed in claim 5, wherein the characteristic time duration of the stretcher is fractionally greater than that of the associated period filter.

9. A periodicity analyzer as claimed in claim I, wherein between each pair of successive period filters there is inserted an OR gate the input of which is connected to the outputs of two period filters which precede the OR gate.

10. A periodicity analyzer as claimed in claim 1, wherein each period filter and each stretcher comprises an electronic circuit arrangement which includes at least a first and a second transistor, the collector of the first transistor being connected to the base of the second transistor, the emitters of the two transistors being interconnected and a capacitor being included between the base of the first transistor and the collector of the second transistor, the base of the first transistor being connected through a differentiating network to an input terminal, while the collector of the second transistor is connected to a Schmitt trigger.

H. A periodicity analyzer as in claim I, wherein the outputs of all the stretchers are connected to an indicator which locates that period filter the characteristic time duration of which is greater than the period of the quasi-periodic signal to be analyzed.

12. A periodicity analyzer as claimed in claim 11, wherein each stretcher is connected to an indicator element which substantially consists of an AND gate the input of which is connected to the output of the associated stretcher and also, through an inverting amplifier, to the output of the next subsequent stretcher.

13. A periodicity analyzer as claimed in claim ll, wherein all the indicator elements are connected to a digital analog converter. 

1. A periodicity analyzer for a quasi-periodic signal, for example speech, comprising a detection circuit means for converting the signal into a plurality of pulses the positions of which correspond to those of the peaks of the quasi-periodic signal, the detection circuit comprising a peak detector the time constant of which is substantially equal to the shortest period of the quasi-periodic signal to be detected; and a cascade arrangement of a plurality of period filter means connected to the detection circuit, the filter means having mutually different characteristic time durations, each filter means comprising means responsive To input pulses separated by an interval greater than the characteristic time duration for providing output pulses having a pulse width equal to the time duration and responsive to a series of input pulses separated by an interval less than the characteristic time duration for providing an output pulse having a pulse width equal to the characteristic time duration plus the total time elapsed between the first and last pulse of the series.
 2. A periodicity analyzer as claimed in claim 1, wherein the characteristic time durations of the greater part of the period filters have ascending values.
 3. A periodicity analyzer as claimed in claim 1, wherein the characteristic time durations of the period filters differ from one another by not more than 1/10 in ascending order.
 4. A periodicity analyzer as claimed in claim 1, further comprising with substantially each of the period filters an associated auxiliary circuit means for distinguishing the main periods from the sub-periods of the quasi-periodic signal to be analyzed.
 5. A periodicity analyzer as claimed in claim 4, wherein each auxiliary circuit consists of a stretcher of a design similar to that of a period filter and of an input AND gate connected to the output of the associated period filter and to the output of the preceding period filter.
 6. A periodicity analyzer as claimed in claim 5, wherein in the circuit connecting the input AND gate to the output of the associated period filter there is included an AND gate the input of which is connected to an OR gate which is connected to the preceding stretcher, to the succeeding stretcher and to the associated stretcher.
 7. A periodicity analyzer as claimed in claim 6, wherein to the said OR gate there is applied a starting signal at the instants at which none of the stretchers delivers an output signal.
 8. A periodicity analyzer as claimed in claim 5, wherein the characteristic time duration of the stretcher is fractionally greater than that of the associated period filter.
 9. A periodicity analyzer as claimed in claim 1, wherein between each pair of successive period filters there is inserted an OR gate the input of which is connected to the outputs of two period filters which precede the OR gate.
 10. A periodicity analyzer as claimed in claim 1, wherein each period filter and each stretcher comprises an electronic circuit arrangement which includes at least a first and a second transistor, the collector of the first transistor being connected to the base of the second transistor, the emitters of the two transistors being interconnected and a capacitor being included between the base of the first transistor and the collector of the second transistor, the base of the first transistor being connected through a differentiating network to an input terminal, while the collector of the second transistor is connected to a Schmitt trigger.
 11. A periodicity analyzer as in claim 1, wherein the outputs of all the stretchers are connected to an indicator which locates that period filter the characteristic time duration of which is greater than the period of the quasi-periodic signal to be analyzed.
 12. A periodicity analyzer as claimed in claim 11, wherein each stretcher is connected to an indicator element which substantially consists of an AND gate the input of which is connected to the output of the associated stretcher and also, through an inverting amplifier, to the output of the next subsequent stretcher.
 13. A periodicity analyzer as claimed in claim 11, wherein all the indicator elements are connected to a digital analog converter. 